The law also allows a sales-tax increase of 2 cents per $10 purchase. Sound Transit and Metro each collect 9 cents per $10 now.

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“We’re not going to be left in a situation where we’re not going to give voters the ability to save the system,” County Councilmember Larry Phillips, of Magnolia, said Thursday at a Metro news conference

The notion that the county might act on its own to raise car fees might be just a brainstorm, or bluster, meant to push state lawmakers — now in a special session — to act.

County officials didn’t give a deadline for going it alone. But they said time is running out, because in December they will officially send the first proposal for cuts, for Highway 99 routes.

Earlier this year, the House passed and the Senate spurned a measure allowing King County to seek a car-tab tax of $150 per $10,000 of vehicle value, split 60 percent for transit and 40 percent to city and county roads.

County Executive Dow Constantine and other leaders have warned for months that without new money, Metro would phase in cuts of 17 percent over the next two years.

General Manager Kevin Desmond said Metro should be growing instead, to deal with full buses and what he called insatiable demand.

Metro on Thursday updated its “What’s at Risk” website to illustrate how service cuts would affect highway corridors and neighborhoods. One example: 13 percent of buses passing Eastgate on Interstate 90 could disappear.

In all, 74 of the 214 routes would be deleted, the site says, while other buses would grow more crowded or run less often.

Metro forecasts that 50,000 of its 400,000 weekday passengers would stop riding, and some would switch to cars.

A temporary $20 car-tab fee and state aid for about 150 daily bus runs on Alaskan Way Viaduct routes expire in mid-2014, while Metro says it never fully recovered from losses in the recession.

The first cuts would be to the viaduct routes, starting June 6, said Desmond.

Aurora Avenue North and Southcenter routes would be mostly spared, but severe cuts would happen in several areas including Ballard, Kenmore, Lake City, Issaquah, the Snoqualmie Valley, the Kingsgate area near Kirkland, and Auburn.

Sung Yang, chief of staff for Constantine, said he’d prefer the Legislature pass an emergency bill to allow King County to raise funds for transit and local roads.

That way, he said Metro could make service decisions its passengers can trust, rather than be tied to a roads package that might be delayed, or even be dependent on a statewide highway ballot measure.